Ace Knute Johnson was delivered by C-Section in a Los Angeles hospital on Sunday, according to US Weekly, who confirmed the news with Simpson's rep.
This is the singer's second child with her fiancé, Eric Johnson, a former NFL player. Back in March, the 32-year-old former MTV star accidentally revealed the sex of her baby during an appearance on "Jimmy Kimmel Live" as she described her difficult pregnancy.
"The crazy thing is I never knew a wiener could make me nauseous," she said laughing before immediately adding, "Well, I guess I just told the world that I'm having a boy!"
Most recently Simpson appeared as a mentor on the NBC style competition show "Fashion Star." The singer, actress and mom is next slated to star in an upcoming, semi-autobiographical NBC comedy. "I am so excited to work with Ben [Silverman] and NBC again, this time on a scripted comedy," Simpson told The Hollywood Reporter earlier in the year.
"I often find myself thinking that no one could ever make up the things that actually happen in my life — so between the real-life elements and a great team of writers, I think we'll have people laughing!"
Simpson and Johnson were engaged in the fall of 2010 but eventually delayed their wedding because of her pregnancies.